U.S. patent number 4,487,910 [Application Number 06/578,733] was granted by the patent office on 1984-12-11 for process for polyurethane prepolymers containing terminal isocyanate groups and having a reduced residual monomer content.
This patent grant is currently assigned to Henkel Kommanditgesellschaft auf Aktien. Invention is credited to Hans Bauriedel.
United States Patent |
4,487,910 |
Bauriedel |
December 11, 1984 |
Process for polyurethane prepolymers containing terminal isocyanate
groups and having a reduced residual monomer content
Abstract
Process for the production of polyurethane prepolymers based on
monocyclic and dicyclic diisocyanates, in which a prepolymer based
on a dicyclic diisocyanate is produced in a prepolymer based on a
monocyclic diisocyanate of comparatively lower reactivity. The
products obtained by this process contain terminal isocyanate
groups and are distinguished by a reduced content of free monomeric
diisocyanates.
Inventors: |
Bauriedel; Hans (Duesseldorf,
DE) |
Assignee: |
Henkel Kommanditgesellschaft auf
Aktien (Duesseldorf, DE)
|
Family
ID: |
6191764 |
Appl.
No.: |
06/578,733 |
Filed: |
February 9, 1984 |
Foreign Application Priority Data
|
|
|
|
|
Feb 25, 1983 [DE] |
|
|
3306559 |
|
Current U.S.
Class: |
528/65;
156/331.4; 528/52; 528/67 |
Current CPC
Class: |
C08G
18/10 (20130101); C08G 18/722 (20130101); C08G
18/7607 (20130101); C08G 18/8009 (20130101); C09J
175/04 (20130101); C08G 18/10 (20130101); C08G
18/758 (20130101); C08G 18/10 (20130101); C08G
18/70 (20130101); C08G 18/10 (20130101); C08G
18/40 (20130101); C08G 18/10 (20130101); C08G
18/7671 (20130101) |
Current International
Class: |
C08G
18/00 (20060101); C09J 175/04 (20060101); C08G
18/10 (20060101); C08G 18/72 (20060101); C08G
18/80 (20060101); C08G 18/76 (20060101); C08G
018/10 () |
Field of
Search: |
;528/65,52,67
;156/331.4 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3839491 |
October 1974 |
Gamero et al. |
4038239 |
July 1977 |
Coyner et al. |
4111865 |
September 1978 |
Seefried et al. |
4423200 |
December 1983 |
Ganster et al. |
|
Foreign Patent Documents
Other References
(cf. H. G. Elias, Makromolekule, Huthig & Wepf, Basel, 4th
Edition 1981) Heidelbert, New York..
|
Primary Examiner: Welsh; Maurice J.
Attorney, Agent or Firm: Szoke; Ernest G. Millson, Jr.;
Henry E. Greenfield; Mark A.
Claims
What is claimed is:
1. A process for the preparation of a polyurethane prepolymer
containing terminal isocyanate groups from both monocyclic and
dicyclic diisocyanates comprising the steps of
a. reacting a monocyclic diisocyanate with a polyhydric alcohol in
an OH:NCO ratio of less than 1 to form a prepolymer; and
b. reacting a dicyclic diisocyanate with a polyhydric alcohol in an
OH:NCO ratio of less than 1, wherein the reaction is carried out in
the prepolymer prepared in step a.
2. A process in accordance with claim 1 wherein the monocyclic
diisocyanate is tolylene diisocyanate, isophorone diisocyanate, or
a mixture of the foregoing.
3. A process in accordance with claim 1 wherein the dicyclic
diisocyanate is 4,4'-diphenyl methane diisocyanate,
4,4'-dicyclohexyl methane diisocyanate, or a mixture of the
foregoing.
4. A process in accordance with claim 2 wherein the dicyclic
diisocyanate is 4,4'-diphenyl methane diisocyanate,
4,4'-dicyclohexyl methane diisocyanate, or a mixture of the
foregoing.
5. A process in accordance with claim 1 wherein in step a. the
OH:NCO ratio is between about 0.4 and about 0.8.
6. A process in accordance with claim 5 wherein the OH:NCO ratio in
step a. is between about 0.5 and about 0.7.
7. A process in accordance with claim 1 wherein in step b. the
OH:NCO ratio is between about 0.65 and about 0.80.
8. A process in accordance with claim 7 wherein the OH:NCO ratio in
step b. is between about 0.70 and about 0.75.
9. A process in accordance with claim 5 wherein in step b. the
OH:NCO ratio is between about 0.65 and about 0.80.
10. A process in accordance with claim 1 wherein the polyhydric
alcohols in steps a. and b. are primary or secondary aliphatic
alcohols containing from 2 to 6 hydroxyl groups.
11. A process in accordance with claim 10 wherein the alcohols
contain from 2 to 4 hydroxyl groups.
12. a process in accordance with claim 10 wherein the alcohols are
diols containing from 2 to 5 carbon atoms, triols containing from 3
to 6 carbon atoms, tetraols containing from 4 to 8 carbon atoms, or
--OH functional polyesters having molecular weights of from about
200 to about 10,000.
13. A process in accordance with claim 1 wherein the dicyclic
diisocyanate employed in step b. is employed in a quantity of from
about 5 to about 80% by weight, based on the total quantity of
diisocyanates employed in the process.
14. A process in accordance with claim 1 wherein step a. and step
b. are each carried out at a temperature in the range of from about
50.degree. C. to about 100.degree. C.
15. A process in accordance with claim 14 wherein said temperature
is in the range of from about 60.degree. C. to about 80.degree.
C.
16. A process in accordance with claim 10 wherein said alcohols are
polyether polyols having a molecular weight of from about 100 to
about 5000.
17. A process in accordance with claim 16 wherein the polyether
polyols are polyoxyethylene or polyoxypropylene polyols.
18. A polyurethane prepolymer prepared in accordance with the
process of claim 1.
19. A method for bonding plastic materials together comprising
contacting the plastic materials to be bonded together with the
polyurethane prepolymer prepared by the process of claim 1 in the
presence of moisture or a hardener composition or both and joining
the plastic materials together until bonding is achieved.
20. A method in accordance with claim 19 wherein the bonding is
carried out at a temperature in the range of from about 70.degree.
to about 120.degree. C.
Description
This invention relates to a process for the production of
polyurethane prepolymers based on monocyclic and dicyclic
diisocyanates. The products obtained by the process of the
invention contain terminal isocyanate groups and are distinguished
by a reduced content of free monomeric diisocyanates.
BACKGROUND OF THE INVENTION
Polyurethane prepolymers containing terminal isocyanate groups have
been known for some time. They can readily be reacted with suitable
hardeners--generally polyhydric alcohols--to form high polymers.
Polyurethane prepolymers have acquired significance in numerous
fields, including for example sealing compounds, lacquers and
adhesives.
To obtain polyurethane prepolymers containing terminal isocyanate
groups, it is standard practice to react polyhydric alcohols with
an excess of diisocyanates. It is generally known in polymer
chemistry that, in this reaction, the molecular weight can be at
least approximately controlled through the ratio of hydroxyl groups
to isocyanate groups. Thus, products of very high molecular weight
are formed where the molar ratio is exactly 1:1, whereas, on a
statistical average, adducts of 2 molecules of isocyanate with 1
molecule of diol are formed where the molar ratio of OH to NCO is
1:2. On the strength of this knowledge, it is possible for those
skilled in this art to custom-make polyurethane prepolymers
containing terminal isocyanate groups with average molecular
weights varying within wide limits. However, the products formed
show a more or less wide molecular weight distribution, as is
normally the case with polymers. In particular, a certain amount of
the component which is used in excess remains unreacted at the end
of the reaction, irrespective of the reaction time. At the same
time, the content of unreacted diisocyanates, referred to herein as
residual monomers, increases with the excess of this component in
the reaction mixture (cf. H. G. Elias, Makromolekule, Huthig &
Wepf, Basel, 4th Edition 1981, pages 487 et seq.).
In numerous applications of polyurethane prepolymers, problems are
presented by the presence of residual monomers. Thus, volatile
diisocyanates, such as tolylene diisocyanate for example,
necessitate particular precautionary measures on an industrial
hygiene level, even when they are present in the prepolymers in
quantities of only 0.5 to 5% by weight. On the other hand,
involatile diisocyanates can cause problems through migration in
the field of bonding. Problems such as these can arise, for
example, in the sealing of bonded film-to-film laminates.
To reduce the residual monomer content, it is possible in the case
of volatile diisocyanates to remove these diisocyanates from the
prepolymers by thin-layer distillation under reduced pressure and
at an elevated temperature in the range of from about 80.degree. to
150.degree. C. However, this process is complicated and
occasionally gives unsatisfactory results. In addition, it is
confined to volatile diisocyanates. In the case of nonvolatile
diisocyanates, such as the dicyclic diisocyanates, distillation has
little to offer. Accordingly, products of relatively high molecular
weight are generally used; products such as these having a
relatively low residual monomer content because of the theoretical
relationships explained at the beginning. Since they also have
relatively high viscosities, it is standard practice to use the
prepolymers in solution in organic aprotic solvents. However, the
use of organic solvents is ecologically unfavorable and is no
longer acceptable in numerous applications.
DESCRIPTION OF THE INVENTION
An object of the present invention is to provide a process for the
production of polyurethane prepolymers containing terminal
isocyanate groups and based on monocyclic and dicyclic
diisocyanates, in which the products obtained have a reduced
residual monomer content despite the fact that the starting molar
ratio of OH groups to NCO groups is less than 1.
Accordingly, the present invention relates to a process for the
production of polyurethane prepolymers containing terminal
isocyanate groups from monocyclic and dicyclic diisocyanates,
wherein, in a first step, a monocyclic diisocyanate is reacted with
a polyhydric alcohol in an OH:NCO ratio of less than 1 and, in the
prepolymer thus formed, a dicyclic diisocyanate is reacted with a
polyhydric alcohol in an OH:NCO ratio of less than 1; the reactions
optionally being carried out in the presence of the usual catalysts
and/or at elevated temperatures.
The invention utilizes the fact that polyurethane prepolymers
containing terminal isocyanate groups have a comparatively low
viscosity when they are derived from a monocyclic diisocyanate.
Accordingly, a prepolymer such as this is produced in a first
reaction step in which the starting molar ratio is selected so that
the product formed is still liquid, at least at the reaction
temperature. In a second reaction step, the reaction of a
polyhydric alcohol with a dicyclic diisocyanate is carried out in
this liquid prepolymer as "solvent" or reactive diluent. In
addition, the process of the invention utilizes the greater
reactivity of the isocyanate groups in the dicyclic diisocyanates
compared with the isocyanate groups in the prepolymers which are
derived from the monocyclic diisocyanates. In this way, the product
obtained in the first step (reactive diluent) reacts minimally, if
at all, with the polyol component added in the second step. Since,
on the one hand, the residual monomer content of the reactive
diluent undergoes a further reduction during the second reaction
step, especially since the monocyclic diisocyanates present as
residual monomers in the reactive diluent contain isocyanate groups
of higher reactivity than the reactive diluent itself, and since on
the other hand relatively high molecular weight prepolymers having
a low content of residual monomers, which would not be processible
without reactive diluent, prepolymer mixtures having a greatly
reduced residual monomer content are obtained overall. Monocyclic
diisocyanates particularly suitable for carrying out the process of
the invention are aromatic compounds which contain two isocyanate
groups of different reactivity. In this connection, 2,4-tolylene
diisocyanate for example is of considerable commercial
significance. Aliphatic, cyclic diisocyanates, such as isophorone
diisocyanate for example, are also suitable. Among the aliphatic
compounds, those containing isocyanate groups of different
reactivity are again preferred.
The monocyclic diisocyanates can be reacted with a large number of
different polyhydric alcohols. Aliphatic alcohols containing from 2
to 6, preferably from 2 to 4 hydroxyl groups per molecule are
suitable for use in this step. Although both primary and secondary
alcohols can be used, secondary alcohols are preferred. Preferably,
when diols are employed herein they contain from 2 to 5 carbon
atoms; for triols, 3 to 6 carbon atoms; and for tetraols, 4 to 8
carbon atoms. It is also possible to use the reaction products of
low molecular weight polyhydric alcohols with alkylene oxides
containing up to 4 carbon atoms. Suitable reaction products such as
these are, for example, the reaction products of ethylene glycol,
propylene glycol, the isomeric butane diols or hexane diols, with
ethylene oxide, propylene oxide and/or butene oxide. It is also
possible to use the reaction products of trifunctional alcohols,
such as glycerol, trimethylol ethane and/or trimethylol propane, or
higher alcohols, such as pentaerythritol for example, or sugar
alcohols with the above-mentioned alkene oxides.
Polyether polyols having a molecular weight of from about 100 to
about 5000 are particularly suitable.
Thus, it is possible--depending on the required molecular
weight--to use adducts of only a few moles of ethylene oxide and/or
propylene oxide per mole, up to more than 100 moles of ethylene
oxide and/or propylene oxide, with low molecular weight polyhydric
alcohols. Other polyether polyols can be obtained by the
condensation of, for example, glycerol or pentaerythritol with the
elimination of water. In addition, polyols of the type commonly
used in polyurethane chemistry formed by the polymerization of
tetrahydrofuran can be employed. Of the polyether polyols mentioned
above, the reaction products of polyhydric low molecular weight
alcohols with propylene oxide and the conditions under which at
least some secondary hydroxyl groups are formed are particularly
suitable. Other suitable polyether polyols are described, for
example, in German Application No. 25 59 759.
In addition, polyester polyols having a molecular weight of from
about 200 to about 10,000 are suitable for reaction with the
monocyclic diisocyanates. In a first embodiment, it is possible to
use polyester polyols of the type obtained by reacting low
molecular weight polyhydric alcohols, particularly ethylene glycol,
propylene glycol, glycerol or trimethylol propane, with from about
1 to about 50 moles of caprolactone. Other suitable polyester
polyols can be produced by polycondensation. Thus, dihydric and/or
trihydric alcohols can be condensed with a sub-equivalent quantity
of dicarboxylic acids and/or tricarboxylic acids or reactive
derivatives thereof to form polyester polyols. Dicarboxylic acids
suitable for this purpose are succinic acid, and its higher
homologs containing up to 12 carbon atoms and also unsaturated
dicarboxylic acids, such as maleic acid or fumaric acid, and
aromatic dicarboxylic acids, particularly the isomeric phthalic
acids. Citric acid and trimellitic acid are suitable tricarboxylic
acids. Polyester polyols particularly suitable for the purposes of
the invention are polyester polyols of the above-mentioned
dicarboxylic acids and glycerol which have a residual content of
secondary-OH groups.
To obtain reaction products of monocyclic diisocyanates with
polyhydric alcohols which may be used as solvent or "reactive
diluent" in the second reaction step in accordance with the
invention, it is important to maintain a certain ratio between
hydroxyl groups and isocyanate groups. Thus, products of
sufficiently low viscosity are formed when the number of --OH
groups divided by the number of isocyanate groups amounts to
between about 0.4 and about 0.8 and preferably to between about 0.5
and about 0.7.
As stated above, to carry out the second step of the process of the
invention, dicyclic diisocyanates are reacted with polyhydric
alcohols in the prepolymers obtained in the first step. The
dicyclic diisocyanates are used in a quantity of from about 5 to
about 80% by weight, preferably in a quantity of from about 5 to
about 60% by weight and, more preferably, in a quantity of from
about 10 to about 40% by weight, based on the total quantity of
diisocyanates in steps 1 and 2. In the second reaction step, the
molar ratio of hydroxyl groups to isocyanate groups, expressed as
the quotient of the --OH groups divided by isocyanate groups, is
between about 0.65 and about 0.80 and preferably between about 0.70
and about 0.75.
In selecting the dicyclic diisocyanates, it is important for the
reactivity of their isocyanate groups to hydroxyl groups to be
higher than that of the terminal isocyanate groups in the reactive
diluent. Accordingly, diaryl diisocyanates are particularly
suitable. 4,4'-diphenyl methane diisocyanate and/or substituted
4,4'-diphenyl methane diisocyanates are preferred.
4,4'-dicyclohexyl methane diisocyanate, which can be viewed as a
hydrogenation product of the former, can be employed although it is
somewhat less suitable.
The polyhydric alcohols given above with respect to the first
reaction step can also be used for the reaction of the dicyclic
diisocyanates. However, it is also possible to use low molecular
weight polyhydric alcohols such as, for example, ethylene glycol,
propylene glycol, their oligomers, the isomeric butane diols or
hexane diols, trifunctional alcohols, such as glycerol, trimethylol
ethane or trimethylol propane, and higher alcohols, such as
pentaerythritol, and/or sugar alcohols.
To carry out the process of the invention, it is also preferred to
react the diisocyanates with the polyhydric alcohols at an elevated
temperature. Suitable temperatures are in the range of from about
50.degree. to about 100.degree. C., preferably from about
60.degree. to about 80.degree. C. In one particularly preferred
embodiment, the first step is carried out at a temperature of from
about 50.degree. to about 90.degree. C. and over a period of from
about 2 to about 20 hours. The starting materials for the second
reaction step are then added at temperatures of from about
60.degree. to about 80.degree. C., homogenized for 15 minutes and
the reaction mixtures subsequently left standing. The reaction is
over when there is no further reduction in the number of isocyanate
groups. This can be analytically determined by titrating the
isocyanate groups and is generally achieved after 2 to 5 days at
room temperature.
The products of the present process have a substantially reduced
content of free, monomeric, monocyclic and also dicyclic
diisocyanates. Thus, in cases where the prepolymers are applied
over large areas at elevated temperatures, i.e. in the range of
from about 70.degree. to about 120.degree. C., preferably from
about 80.degree. to about 100.degree. C., no problems are caused by
volatile monocyclic diisocyanates.
The prepolymers according to the invention can be used either as
such or in solution in organic solvents for bonding plastics and,
more particularly, for laminating plastic films. In this case,
standard hardeners, such as polyhydric alcohols of relatively high
molecular weight (2-component systems) can be added or surfaces of
known moisture content can be directly bonded using the products of
the invention. Film-to-film laminates produced using the products
of the invention are safe to heat-seal. This may be attributable to
the reduced content of migratable low molecular weight products in
the prepolymers.
The invention will be illustrated by the following examples, which
are not given for purposes of limitation.
EXAMPLES
Example 1
600 g of polypropylene glycol (0.31 mole)--OH number 58,360 ppm of
water) were mixed with 94.06 g of 2,4-tolylene diisocyanate (48.25%
NCO, 0.54 mole) in a reaction vessel equipped with a stirrer. The
reaction mixture was heated to 65.degree. C. and left at that
temperature for 16 hours. A polyurethane prepolymer having a
viscosity of 6.0 Pa.s at 23.degree. C. and a residual monomer
content of 0.2% (2,4-tolylene diisocyanate) was formed. The
prepolymer had an overall NCO content of 2.62%. 171.61 g of
4,4'-diphenyl methane diisocyanate (33.5% NCO, 0.686 mole) were
introduced into the prepolymer and melted by heating to 70.degree.
C. After the addition of 291.11 g of polytetrahydrofuran
(polytetramethylene glycol, OH number 176, 398 ppm of water, 0.456
mole) the mixture was homogeneously stirred at 70.degree. C. and
then left standing without heating. After storage for 1 week, the
mixed prepolymer formed had a viscosity of 2.50 Pa.s at 75.degree.
C. and residual isocyanate monomer contents of 0.01% of
2,4-tolylene diisocyanate (below the detection limit) and 2.4% of
4,4-diphenyl methane diisocyanate. The titratable NCO-content
amounted to 3.17%.
Example 2
The procedure of this example was similar to that of Example 1,
except that an aliphatic adipic acid polyester was used in the
first step (quantity weighed in 500 g, OH number 56, 334 ppm of
water). A reactive diluent having a viscosity of 7.85 Pa.s at
75.degree. C. was obtained by reaction at 65.degree. C. with 78.4 g
of 2,4-tolylene diisocyanate (0.45 mole). In the second reaction
step, 92.54 g of 4,4'-diphenyl methane diisocyanate (0.37 mole)
were melted at 70.degree. C. in the reactive diluent thus obtained
and the resulting melt reacted for 2 hours at 70.degree. C. with
155.34 g of polytetramethylene glycol (OH number 176, 398 ppm of
water, 0.244 mole). The reaction mixture was then left standing for
2 days, after which the following values were determined: %
NCO=2.77, % monomeric 2,4-tolylene diisocyanate<0.03, %
monomeric 4,4'-diphenyl methane diisocyanate: 1.5, viscosity: 7.15
Pa.s at 90.degree. C.
Example 3
800 g of a prepolymer were produced in the same way as described in
Example 1. 93.0 g of 4,4'-diphenyl methane diisocyanate (0.37 mole)
and 107.0 g of polypropylene glycol (145 ppm of water, 0.246 mole)
were stirred for 2 hours at 70.degree. C. in the prepolymer. The
reaction mixture was then left standing at room temperature for 3
days, after which the following analytical values were determined:
% monomeric 2,4-tolylene diisocyanate<0.01, monomeric
4,4'-diphenyl methane diisocyanate: 1.3.
Example 4
A polyurethane prepolymer was initially produced as in Example 2
from an aliphatic adipic acid polyester and 2,4-tolylene
diisocyanate. 276 g of 4,4'-diphenyl methane diisocyanate (1.103
mole) and 524 g of an adipic acid polyester (aliphatic, OH number
154, 226 ppm of water, 0.72 mol) were then stirred for 2 hours at
70.degree. C. in 1200 g of this polyurethane prepolymer. The
reaction mixture was then left standing at room temperature for 2
days, after which the following analytical data were determined: %
NCO=2.83, % monomeric 2,4-tolylene diisocyanate: 0.02, % monomeric
4.4'-diphenyl methane diisocyanate: 2.3, viscosity: 9.26 Pa.s at
90.degree. C.
The residual monomer contents were determined by gel
chromatography. The percentages are based on the total quantity of
prepolymer.
* * * * *